1. The Field of the Invention
The present invention relates to materials for gettering, or sorbing, various gases. More particularly, the present invention relates to getter materials having low applied activation temperatures, methods for making such materials, and devices including such materials.
2. The Relevant Art
Getter materials are widely employed in applications requiring a high static vacuum or for the purification of inert gases. These materials operate by sorbing molecules of reactive gases which are thus secured and removed from the environment to be evacuated or from the gas to be purified. Getters are subdivided into two main classes: evaporable getters and non-evaporable getters ("NEGs"). Evaporable getters typically include the alkaline earth metals calcium, strontium and especially barium. Non-evaporable getters generally include titanium, zirconium or alloys thereof with one or more metals selected from amongst aluminum and the metals of the first transition row. Both getter types require activation by heating for their operation to remove from the getter surface various oxides, carbides, and nitrides that otherwise prevent the gaseous species to be removed from being sorbed on the getter's surface. Indeed, because of their high reactivity towards atmospheric gases, getters are manufactured and traded in an inactive form and require a suitable activating heat-treatment once they are arranged into the volume to be evacuated.
Evaporable getters are commonly employed in cathode ray tubes ("CRTs") such as used for television and computer monitor screens. In these applications, barium (Ba) is almost always used as the getter metal and is typically deposited as a metal film onto an inner wall of the cathode tube. The activation step resides in the evaporation of the barium metal onto the inner surface(s) of the CRT. Barium evaporation is usually carried out either by heating a barium-containing composition from outside of the cathodic tube, or by exposing the composition to radio-frequency radiation. Generally , the barium-containing composition is BaAl.sub.4 which is combined with nickel (Ni) powder. At a temperature of about 850.degree. C. the nickel reacts with the aluminum and the heat generated by such a reaction causes barium to evaporate, according to a so-called "flash" phenomenon.
NEGs are used for several applications, including high-vacuum getter pumps, to maintain vacua in jackets evacuated for thermal insulation purposes, and as gas scavengers inside fluorescent lamps. Often the NEGs are formed into getter bodies of compressed and sintered powders, charged into containers, or laminated onto metal strips. As described above, an activation treatment is generally performed prior to use to remove the thin layer of oxides, carbides and nitrides that accumulate on the surface of the NEG material when the material is exposed to air. Activation by heat-treatment allows these species to migrate towards the getter particle core; thus exposing the metal surface of the particle which is active in gas chemisorption. The activation temperature depends on the composition of the NEG, and can vary from about 350.degree. C., for an alloy having the percentage weight composition of 70% zirconium (Zr)--24.6% vanadium (V)--5.4% iron (Fe), sold under the trade name St 707 by SAES Getters S.p.A. of Lainate (MI) Italy, to about 900.degree. C. for an alloy having a percentage weight composition of 84% Zr--16% Al which is also manufactured and traded by SAES Getters under the trade name St 101.RTM.. In general, the activation is performed after the getter material has been placed in its operating environment and the surrounding atmosphere has been at least partially evacuated.
Because both evaporable and non-evaporable getter materials must be activated by heating after the materials are placed into their operating environment, which may contain heat-sensitive instruments or materials, it is often desirable to choose materials that require a relatively low applied heat for activation. For instance, in the case of vacuum jackets made from steel (which have largely replaced the traditional glass vacuum bottles in the market), the steel surface is oxidized by the getter activation step; thus requiring a mechanical cleaning operation to remove the discoloration caused by the accumulated oxidant. This could be avoided if the getter activation could be performed by heating the getter material at a temperature lower than about 300.degree. C. Finally, by working at a lower applied temperature it is possible to use equipment having less complexity and expense than equipment used for high temperature applications and with greater savings in power costs.
However, sometimes getter materials that can be activated at an applied temperature that is lower than the actual activation temperature, but still greater than a minimum temperature, are preferred. In some manufacturing processes, for instance, in the manufacture of television tubes, it would be desirable to have a getter that can be activated at an applied temperature of less than about 850.degree. C. (which is required by the barium evaporable getters presently on the market), but which is not activated during the sealing of the two glass portions forming the cathode tube, which is performed at a temperature of about 450.degree. C., in order to avoid barium evaporation while the device is still open.
The published Japanese patent application Kokai 8-196899 discloses a non-evaporable getter system consisting of a mixture of powders of titanium (Ti), titanium oxide (TiO.sub.2) and barium peroxide (BaO.sub.2). Both oxides are expected to partially oxidize the titanium metal to form the intermediate oxide Ti.sub.2 O.sub.5 ; the heat produced by this reaction should then activate the residual titanium. Preferably from 3 to 5% of silver powder is added to the mixture in order to render the system temperature more uniform. According to this document the disclosed mixture should become activated at a temperature of from 300 to 40020 C. However this solution is not satisfactory. First, the mentioned application discloses only the Ti-TiO.sub.2 --BaO.sub.2 system and the gettering capacity of titanium is not very high. Second, titanium oxide is an extremely stable compound which does not release oxygen and in any case, even if this occurred, the oxygen would merely be transferred from titanium atoms to other titanium atoms with a heat balance of zero; hence without any heat release useful for activating the getter system. Finally the document gives no example demonstrating the activation of titanium getter. In fact, as discussed below, tests performed using the descriptions in this publication failed to show any lowering of applied activation temperature.